1. Field of the Invention
This invention is related to vaccines for fish. In particular, the invention is directed to a vaccine for the prevention of Enteric Septicemia.
2. Background Art
An increase in channel catfish production from less than 6 million pounds in 1970 to more than 200 million pounds in 1985 has led to an increase in accompanying mortality loss due to infectious diseases. Since its first discovery in 1976, Edwardsiella ictaluri (EI) has become the leading cause of bacterial fish mortality in commercially raised channel catfish.
EI has been isolated from channel catfish, Ictalurus punctatus (Hawke 1979); the green knifefish, Eigmounia viresceus (Kent and Lyons 1982); danio, Danio devario (Lobb and Rhoades 1987); rosy barb, Puntius couchouius (Humphrey et al. 1986); walking catfish, Clarius batrachus (Kasornchandra et al. 1986); white catfish, Ictalurus catus (Newton et al. 1988); and harlequin tetra, Rosbara heteromorpha (Reid and Boyle 1989). This organism causes an acute septicemia (Enteric Septicemia (ES)) in fast-growing fish. Externally, EI can cause hemorrhages of the skin around the mouth and throat, pale gills, exopthalmia, lesions of the fontanelle of the frontal bone of the skull, and small cutaneous lesions on the lateral body surface. Internally, lesions may include hypertrophy of the kidney and spleen, hemorrhage and necrosis of the liver, hemorrhage of adipose tissue, the internal wall of the dorsal musculature, and intestine. Death can occur within 96 hours post injection or up to two weeks after water borne challenge (Hawke 1979). Entry of the organism into the catfish has been shown to occur by both olfactory and intestinal routes (Newton et al. 1989; Saeed 1983; Blazer et al. 1985; Miyazaki and Plumb 1985; Shorts et al. 1986).
While Waltman et al. (1986) proposes that chondroitinase may be a possible contributor to virulence, many other possible factors are also set forth. For example, Newton et al. (1988) allude to possible plasmid versus chromosomal mediated virulence factors, noting the longevity of plasmids pEI1 and pEI2 in EI isolates from channel catfish. Blazer et al. (1985) note the need to access the role of the macrophage in the pathogenesis of ES. The virulence factors for the organism, however, remain unknown (Newton et al. (1988)). Likewise, attempts at producing a viable vaccine have been unsuccessful and antibiotic resistant strains of EI are increasing production costs (Starliper 1991).
Consequently, there exists a need for an effective vaccine against ES. This invention satisfies those needs by identifying a primary virulence factor of EI and by providing a viable vaccine and a method of administration.